1. Field of the Invention
This invention relates to a pre-coated metal sheet used for slot-in drive cases in housings of electric equipment and electronic equipment such as liquid-crystal television sets, personal computers and DVD players. In the present invention, the term “pre-coated” is used to mean “coated in advance of press molding”.
2. Description of Related Art
In recent years, development of housings of electric equipment and electronic equipment and development of optical disks such as CDs and DVDs (digital video disks) have been put forward, and slot-in drive cases have begun to become popular which have enabled drive cases to be more space-saving. Such a slot-in system is one having the function that an optical disk is automatically drawn into the drive case by just placing the disk into a slot as the disk stands bare, where the optical disk can more smoothly be taken in and out than the conventional tray system. Recently, pre-coated metal sheets are being employed in such slot-in drive cases. For example, a pre-coated steel sheet is employed which is used to form the drive case and has, on its inner surface, a thin coating of 1 μm or less in thickness which has a good sliding property. Because of its superior sliding property, it is advantageous in that, when the optical disk is slotted in, the edge face of the optical disk comes into contact with the coating film surface at so small resistance as to make a small rubbing noise. However, very small foreign matter stands mixed in the thin coating and there are coating defects usually not coming into question. Hence, the optical disk may be scratched when the optical disk comes into contact with the coating film formed on the inner surface of the drive case. Accordingly, depending on what structure the drive case has, such a pre-coated steel sheet has in some cases been kept from being employed.
The slot-in drive case has a structure that the optical disk tends to be scratched. The space between the optical disk and the inner surface of the drive case is so narrow that it may come about that the optical disk strikes against the inner surface of the drive case in relation to the thickness precision of the optical disk itself and any deflection of the optical disk at the time of its driving. It may also come about that the optical disk comes into contact with the inner surface of the drive case when any accidental impact is applied to the drive during reproduction from the optical disk or during recording on the same. It may further come about that, depending on how the optical disk is placed in, it is slotted into the drive case in the state the optical disk is inclined, so that the optical disk may come into contact with the inner surface of the drive case. The inner surface of the drive case may have various structure, where, of the inner surface of the drive case, the part with which the optical disk has a possibility of coming into contact is a plane surface, and hence the both come into face to-face contact.
Various kinds of optical disks have been developed. Some of them may adversely affect the reproduction and writing of records just when the surface of the optical disk is slightly scratched. Thus, there has been a disadvantage that the optical disk comes not usable if the optical disk is scratched as a result of the contact of the optical disk with the inner surface of the drive case.
In order to prevent the optical disk from coming scratched, it is proposed, as disclosed in Japanese Patent Application Laid-open No. 2003-242703 (Patent Document 1), that a scratch-preventive coating composed basically of a urethane resin and containing at least polyurethane particles and a fluorine resin powder is formed on the surface of any member with which the optical disk has a possibility of coming into contact in an optical-disk unit, to prevent the optical disk from being scratched.
However, the scratch-preventive coating disclosed in Patent Document 1 is a post-coating formed partially by coating carried out after press molding. Such a post-coating must be formed by coating carried out in a stated layer thickness, partially or on the whole for each press-molded product. Hence, there remains a problem that such coating requires so complicated process as to result in a low productivity and make an increase in production cost unavoidable.
Meanwhile, it is difficult for the reason stated below, that the coating material disclosed in Patent Document 1 is diverted to a coating material for pre-coating.
In the scratch-preventive coating disclosed in Patent Document 1, the urethane resin, which has a rich flexibility, is used as a base resin, and hence the coating formed may deform when the optical disk comes into contact with the scratch-preventive coating, so that the area of contact between the coating and the optical disk comes larger than cases in which other resins are used. Such an increase in contact area damages scratch-preventive properties. Accordingly, the polyurethane particles are incorporated in the coating in an amount of as large as from 20 to 150 parts by weight based on 100 parts by weight of the base resin, so as to make the contact area smaller. In the coating containing the polyurethane particles in such a large quantity, the polyurethane particles tend to come off the coating, so that the effect of preventing scratch that is to be brought by the addition of the polyurethane particles may insufficiently be brought out. There has been such a problem
In addition, in what is disclosed in Patent Document 1, the low frictional, fluorine resin powder must be incorporated in the coating as a large component which is in an amount of from 10 to 150 parts by weight based on 100 parts by weight of the base resin. Since such a coating contains the fluorine resin powder in a large quantity, there have been problems that not only an increase in material cost is unavoidable but also it is difficult to wind up the product sheet when it is coiled up.
Moreover, in the coating film disclosed in Patent Document 1, the fluorine resin powder is not liquefied at the temperature where the coating film is baked (temperature: 80° C. to 140° C.) and is kept in the state of softened powder. Hence, any sufficient properties of penetration into the base resin are not obtainable to tend to cause dispersion non-uniformity locally, so that the sliding property may come non-uniform. There has been such a problem as well.
A pre-coated aluminum alloy sheet is also proposed which is a pre-coated aluminum alloy sheet used for drive cases of electronic equipment such as personal computers, and aims to prevent the drive case itself from being scratched at the time of press molding and because of its contact with other component parts (Japanese Patent Application Laid-open No. 2004-98624). Its coating film contains a base resin such as polyester-series resin, beads of an acrylic-series resin or the like, and an inner wax such as polyethylene wax. However, this coating film is what is considered to prevent itself from being scratched, and is not what prevents the other member optical disk from being scratched. More specifically, in this coating film, the beads are added to the base resin to make the coating film strong and further the inner wax is added to provide the coating film with lubricity, and hence this coating film takes it as a basic idea to make itself not easily scratched at the time of press molding. Thus, this coating film takes it into consideration to chiefly satisfy the performance required on the outer-surface side of the drive case, and does not take it into consideration to satisfy the performance required on the inner-surface side of the drive case as in the present invention, i.e., to prevent the other member optical disk from being scratched. The present inventors have evaluated disk scratch-preventive properties in respect of a coating film in which acrylic resin beads and carnauba wax have been added to the polyester resin. As the result, they have confirmed that the optical disk is seriously scratched.